[FFmpeg] Add support for VP8/VP9 codecs

[emptyVoid]

Added vpx feature to ffmpeg to support WebM VP8/VP9 codecs.

Resolves #6449.

[emptyVoid]

Same errors again (see #6729 (comment)):

error: failed to update mingw32 (unable to lock database)
error: failed to update mingw64 (unable to lock database)
error: failed to update msys (unable to lock database)
error: failed to synchronize all databases

[cbezault]

Sorry about those database locking errors, we're working on ironing them out. I just want to make sure that this PR is only meant to enable VP8/VP9 on Windows right?

[emptyVoid]

Yes, enabling VP8/VP9 on Windows only. Should work for other platforms too, though libvpx port currently supports Windows only.

[cbezault]

I'm going to allow this for now because it works for you and doesn't break anything else but the mechanism you are using to detect whether or not we're compiling a debug library will only work on windows. A better fix would have been to work on the upstream port. I'm going to be doing just that for BZip2 to get it to work with ffmpeg without the kind of patching you did so I might revisit this then.